The present invention relates to a foldable propeller for a boat, ship or other water vehicle.
In the field of sailing-boats, the use of so-called foldable propellers is previously known. Normally, such a propeller is adapted to be used with a propulsion engine for propelling the boat forwardly or rearwardly.
A propeller imposes a certain drag force on the sailing-boat when the propeller is not used. For this reason, the propeller can be made foldable, i.e. the blades of the propeller are pivotably arranged on a hub so that they fold together (as a result of the movement of the boat in the water) in the direction of the propeller""s drive shaft, to a position in which they extend generally in the longitudinal direction of the boat. When the propeller is to be used, the blades are unfolded by means of the rotating action of the drive shaft. The propeller blades are normally designed so as to form a streamlined body in the folded-together state, thereby reducing the drag force.
A number of foldable propellers are previously known. For example, WO 93/01972 shows a propeller which comprises at least three blades which are pivotably mounted between an unfolded position and a folded-together position.
A problem which is significant for the previously known foldable propellers is that they either impose significant drag during sailing or do not deliver the required thrust in forward or reverse operation. Propellers having blades that fold together usually give a low drag force due to their streamlined bodies. However, in forward and particularly in reverse operation, the known folding type of propeller has relatively poor propulsive performance.
A particular problem regarding previously known folding propellers concerns the propulsion during reverse operation. High reverse thrust at bollard or near bollard pull condition (i.e. when the propeller operates at a boat speed which is zero or close to zero) is usually achieved by adding weight to the tip of the blade, thus increasing the centrifugal moment about the blade""s pivot axis. In this manner, the opening angle of the blade is increased. However, increasing the weight at the tip of the blades either involves a problem in the form of thick blade sections with poor cavitation properties or in the form of long sections which tend to reduce the propeller efficiency in forward operation of the propeller.
Another problem which is common not only to sailing-boat propellers, but to any propeller operating in a non-uniform velocity field, is the noise and vibrations induced by the propeller. The propeller generates pressure pulses which force the boat""s hull or superstructure to vibrate fiercely and thus to generate unwanted noise. In applications where the propeller""s drive shaft is connected to a high power propulsion means, the risk for high noise levels is significant for previously known folding propellers, since their blades are usually too narrow and blunt in order to avoid cavitation, which is not only a cause of thrust breakdown, but also a major source of noise and vibration.
In known designs, several methods for reducing noise and vibrations exist; for example, increasing the number of blades. A propeller with many blades generates less fluctuating propeller forces than a propeller with fewer blades since the propeller hub acts as an integrator, i.e. the load on individual blades is superimposed by the hub and transferred via the propeller shaft to the hull of the boat.
Noise and vibrations can also be reduced by reducing the pitch at either the blade""s tip or its root, or both. This reduces the blade loading locally and thereby reduces the strength of the tip and hub vortices, which usually induce substantial pressure pulses on the hull.
Moreover, noise and vibrations can generally be reduced by avoiding cavitation or by arranging the propeller with skewed blades. Cavitation is normally avoided by giving the propeller a sufficiently large blade area. Injection of air into vapor cavities is also an effective method for eliminating their erosive behavior and the generation of high frequency noise.
A particular problem related to foldable propellers is possible thrust reduction due to cavitation at high drive shaft power. In known designs, this problem is solved by giving the propeller a sufficiently large blade area, which is accomplished by using long blade sections and/or a large number of blades. However, the blade area cannot be made too large, since this decreases the propeller efficiency and also complicates the folding of the blades.
A general problem related to propellers is to obtain a high forward thrust or propeller efficiency at any speed. The general solution to this problem is a large propeller diameter in combination with a low drive shaft speed. In addition, the radial load distribution of the propeller should be optimum and the blade area should be made large enough in order to avoid cavitation. Furthermore, the blades should have thin cambered sections of the airfoil type.
Another problem which relates to foldable propellers concerns the folding function. Previously known folding propellers having high pitch-diameter ratio may have poor opening characteristics. The reason for this is that the blades of the propeller xe2x80x9cshadowxe2x80x9d each other, i.e. they cover each other more or less completely in the folded-together state. The hydrodynamic moment about the blade""s pivot becomes negative and so large in magnitude when the blades are fully folded that the positive centrifugal moment never becomes large enough to start the opening of the propeller. The known solution to this xe2x80x9cshadowingxe2x80x9d problem is to tilt the blade sideways. However, a major drawback of tilting the blades is that they do not fold as well. This generates a higher drag force during sailing.
A particular requirement relating to foldable propellers is that they should present low drag during sailing. This is generally achieved by giving the propellers a streamlined shape in the folded-together position. The usual low-drag solution is a propeller with a hub of small diameter and two straight narrow blades that fold with the flow during forward motion of the boat. A foldable propeller of this kind is previously known from GB 1416616.
Another problem relating to folding propellers is that the folding mechanism malfunctions on occasion, possibly causing both personal injury and material damage.
In view of the above described deficiencies associated with the designs and implementations of known designs for foldable propellers, the present invention has been developed to alleviate these drawbacks and provide further benefits to the user. These enhancements and benefits are described in greater detail hereinbelow.
The present invention in its several disclosed embodiments alleviates the drawbacks described above with respect to conventionally designed foldable propellers and incorporates several additionally beneficial features.
One object of the present invention is to provide a foldable propeller which solves the above-mentioned problems, in particular the problems regarding high reverse thrust of the propeller and low noise and vibrations. This object is accomplished by configuring a foldable propeller according to the teachings of the present invention, the features of which will be defined in greater detail hereinbelow.
According to a preferred embodiment of the present invention, the propeller presents highly skewed blades, i.e. the blades have a generally curved shape where the leading edge of the inner and outer radii are respectively located substantially aft and forward of the blade""s generator line.
Preferably, the propeller presents a developed blade-area ratio which is greater than approximately 35%; at least in the case where three blades are used. Consequently, the developed blade-area can be said to be greater than approximately 10% xe2x80x9cper bladexe2x80x9d. This gives a very effective and reliable folding function, low noise and vibration levels, less cavitation at high shaft power and a reduced moment about the blade""s generator line. The term xe2x80x9cdevelopedxe2x80x9d blade-area can be defined as the area presented by one surface of a blade if it is xe2x80x9cflattened outxe2x80x9d, i.e. the pitch angle for the blade is zero for each blade section and the resulting area is measured.
The design of a propeller is always a process of finding the best compromise to a set of requirements. In this process, some requirements are given more importance than others. For most previously known propellers, high efficiency in forward operation and cavitation avoidance are the two requirements which are given the highest priority. This is also the case for previously known sailing-boat propellers.
However, the operational characteristics of the foldable propeller of the present invention have been given a completely different order of priority, or weighting. Here, high weight has been given to responsive unfolding and high reverse thrust at bollard or near bollard pull condition. Next, weight has been given to low noise and vibration. Subsequently, weights has been given to cavitation avoidance and high forward thrust. Consequently, the folded propeller of the present invention is designed to provide, in particular, streamlined, low-drag sailing characteristics, responsive unfolding from the streamlined nested or folded configuration, high reverse thrust at bollard or near bollard pull condition and low noise and vibration levels on board.
Noise and vibrations can normally be avoided by using skewed blades which, contrary to conventional straight blades, gradually enter regions with disturbed flow and therefore generate a smoother blade loading history, by means of which a decrease in the amplitude of the load on the blade is achieved. However, in the prior art, no foldable propeller having a substantial skew exists. This is due to the fact that, according to the previously known technology, a skewed propeller is difficult to fold.
It should be noted that in common usage the terms xe2x80x9cboatxe2x80x9d and xe2x80x9cshipxe2x80x9d are normally intended to cover different types of vessels, however, in the present application each should be considered interchangeable for purposes of the claims. Each is also considered to include small boats as well as large ships, or any other vehicle for use in or on water. Furthermore, the invention can be used on boats with or without sails.
The beneficial effects described above apply generally to the exemplary constructions and implementations disclosed herein for a skew-bladed foldable propeller. The specific structures through which these benefits are delivered will be described in detail hereinbelow.